Studies were continued of the significance of the limit to the extractability of erythrocyte membrane cholesterol by plasma lipoproteins. Neither the kinetics of the extraction, nor the maximum amount extractable, are a function of the ages of the cells. Despite the fact that only a portion of the membrane cholesterol is extractable, all of it is exchangeable for (labelled) cholesterol incorporated into lipoproteins. The kinetics of reextraction of cholesterol, from cells in which the initially extractable cholesterol was replaced by labelled cholesterol, confirm the presence of two cholesterol pools and indicate a slow exchange between them. Work was started on the measurement of fluorescence resonance energy transfer between membrane proteins and certain polyene antibiotics, which form fluorescent complexes with cholesterol. This type of measurement would provide a means of determining membrane structural changes involving cholesterol. BIBLIOGRAPHIC REFERENCE: Gottlieb, M. H.: The reactivity of human erythrocyte membrane cholesterol with a cholesterol oxidase. Biochim. Biophys. Acta 466: 422-428, 1977.